From what I have heard, most hobby servos are designed for 6V which is just a little low for the Arduino's regulator.
Servos were originally designed for the R/C hobby world to typically run with 4 series connected AA cells, so they had to be able to work reliably up to 6v or as low as 4.8volts that typical cells at the time available (ni-cads or alkalines). So they certainly run faster/higher torque at 6vdc, they are fine running at regulated +5vdc. I would not try and run them at 7vdc unless the manufacture rated them as such in their datasheet.
Lefty